Business combinations (Acquisition details) (Details) $ in Thousands			3 Months Ended				12 Months Ended
		May 01, 2017 USD ($) Agent States	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Liabilities acquired
Net loss reserves			$ 42,575				$ 42,575
Measurement period adjustments							1,872
Goodwill			229,573				229,573	$ 196,758	$ 196,758
Transaction expenses			0	$ 0	$ 4,427	$ 0	4,427	$ 0	$ 0
Crop Risk Services
Business Acquisition [Line Items]
Number of agents | Agent		1,170
Number of states in which entity operates | States		36
Total purchase price
Total purchase price		$ 185,576
Tangible assets acquired
Cash and cash equivalents		1,653
Premiums receivable		537,383
Prepaid reinsurance premiums		227,157
Other assets		184,216
Tangible assets acquired		950,409
Intangible assets		63,921
Deferred tax arising on Goodwill		6,443
Liabilities acquired
Reinsurance balances payable		294,201
Unearned premiums		406,649
Net loss reserves		42,575
Other liabilities		122,715
Liabilities acquired		866,140
Excess purchase price (goodwill) at acquisition		30,943
Measurement period adjustments	[1]						1,872
Goodwill			$ 32,815				$ 32,815
Crop Risk Services | Trade names
Business Acquisition [Line Items]
Useful lives of acquired intangible assets							10 years
Tangible assets acquired
Intangible assets		9,568
Crop Risk Services | Distribution network
Business Acquisition [Line Items]
Useful lives of acquired intangible assets							10 years
Tangible assets acquired
Intangible assets		52,898
Crop Risk Services | Technology
Business Acquisition [Line Items]
Useful lives of acquired intangible assets							2 years
Tangible assets acquired
Intangible assets		$ 1,455

[1] During the year ended December 31, 2017, the Company recorded tax related measurement period adjustments of $1,872.